Text fix: hammer-on/pull-offs -> hammer-ons/pull-offs

[Pruchniakowski]

Text fix: hammer-on/pull-offs -> hammer-ons/pull-offs
Missing letter 's'; it should be like in other similar cases.

Greetings,
Grzegorz

Resolves: #NNNNN

• I signed the CLA
• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
• The code in the PR follows the coding rules
• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)